Dolby Digital

Dolby Digital, known in Chinese as 杜比数字, is a lossy digital audio coding system developed by Dolby Laboratories; its base bitstream is also known as AC-3. It can carry one to five full-range channels and an optional low-frequency effects channel, and is therefore commonly used for 1.0, 2.0, and 5.1-channel programs. The AC-3 encoding and decoding processes were later adopted by standards bodies such as ATSC and ETSI.

杜比数字 was first commercially deployed in movie theater screenings; *Batman Returns*, released in 1992, was one of the earliest films to adopt this system. In the 35-millimeter film version, digital data was recorded between the perforations while retaining the analog optical soundtrack; if the digital section could not be read, the projection system could switch to the analog soundtrack. Home media versions, however, write the AC-3 bitstream directly to DVDs, broadcast streams, or file containers, no longer using the physical recording method found on film.

AC-3 employs lossy transform coding based on auditory models. Audio is divided into short-term spectral coefficients; the encoder allocates quantization precision based on the masking effect and available bitrate, then organizes the exponent, mantissa, and control information into synchronous frames. The decoder can recover continuous PCM but cannot reconstruct the pre-encoded data bit-for-bit. To handle sudden sound events, the encoding process can vary the transform block length to balance frequency resolution and time resolution. The base specification supports sampling rates of 32, 44.1, and 48 kHz, as well as a range of fixed bit rates from 32 kbit/s to 640 kbit/s. 5.1-channel Dolby Digital in DVD-Video typically uses 384 or 448 kbit/s, while Blu-ray can use up to 640 kbit/s. Higher bitrates provide the encoder with a larger data budget, but the final result is also influenced by the number of channels, program content, encoder version, and mastering.

The “0.1” in 5.1-channel audio refers to a bandwidth-limited low-frequency effects channel, rather than one-tenth of a channel calculated by volume or the number of physical speakers. AC-3 also supports various encoding modes, such as 3/2, 2/1, and dual mono. The channel mode in the bitstream describes how the encoded channels are organized; the actual playback device can downmix them to stereo or mono based on the output configuration.

杜比数字 contains metadata transmitted alongside the audio data. `dialnorm` is used to indicate the reference level for program dialogue; dynamic range control parameters provide compression curves for different playback environments; and center and surround downmix coefficients affect the scaling when a multichannel program is converted to fewer channels. These parameters do not alter the lossless or lossy nature of the audio, but they cause the same bitstream to produce different playback levels and dynamic performance under different device settings.

DVD-Video incorporated Dolby Digital into its widely used standard audio system, and AC-3 has also been extensively adopted by digital television, satellite broadcasting, and video game consoles. Blu-ray players can decode Dolby Digital and allow compatible audio tracks to be stored at a higher upper limit of 640 kbit/s. The compressed audio stream can be transmitted to an external decoder via S/PDIF or HDMI, or converted to PCM within the playback device.

Dolby Digital Plus uses E-AC-3 encoding, an extension of AC-3 that offers more flexible frame structures, higher bit rates, and up to 7.1 discrete channels. Systems that support E-AC-3 can serve older devices through specified conversion or compatibility mechanisms, but AC-3 decoders cannot directly interpret the full E-AC-3 extended data. DD, AC-3, E-AC-3, and DD+ should be distinguished from one another on packaging or in media information.

Dolby TrueHD uses lossless encoding and is not the same type of compression as the lossy Dolby Digital. Some Blu-ray releases include a Dolby Digital-compatible audio track in addition to the TrueHD main audio track; this differs from the structure commonly found in DTS-HD Master Audio, which features a lossy core with a lossless extension within a single audio stream. Dolby Atmos is also not simply another name for AC-3: the home version of Atmos is typically released via Dolby TrueHD, Dolby Digital Plus, or other formats that support spatial metadata.